In recent years, flash memory devices have been widely used in computer related equipment and other electronic appliances as storage devices. The nonvolatile and on-chip programmable capabilities of a flash memory are very important for storing data in many applications. As an example, flash memories are frequently used for the BIOS storage of a personal computer. In addition, the small physical size of flash memories also makes them very suitable for portable applications. Therefore, they have been used for storing programs and data for many portable electronic devices such as cellular phones, digital cameras and video game platforms.
Different from a normal random access memory (RAM) that can be randomly read, erased and programmed on a byte basis, a conventional EPROM-type flash memory features a byte-program and a block-erase capability with each block containing a number of bytes. Because the data within a memory block can not be selected for erasure individually, a flash memory has to erase the data of a whole block of memory cells, i.e., an erase block, and then program the new data byte by byte. The block erase scheme, however, not only is inflexible but also has an undesirable problem called over-erasure. The over-erasure results from the inherent difference between the speed of erasing of each memory cell. Because a large number of cells are erased together, the cells having the fast speed of erasing may be over-erased below 0V, while the cells having slow speed are not successfully erased yet. The over-erased cells will conduct leakage current and cause the malfunction of bit line (BL) sense amplifiers.
To add more flexibility for erasing memory cells, isolate non-selected memory cells and avoid disturbance of data, U.S. Pat. No. 5,548,551 provides a negative voltage decoder for erasing either one memory cell or a block of memory cells for a non-volatile memory. In practical applications, however, it is desired that a small (multiple bytes) and flexible (random and multiple word lines) erase size for the erase operation of a flash memory can be accomplished without any memory disturbance and over-erasure problems.